The term hydrous calcium silicate denotes a crystalline compound formed by the reaction of lime (CaO), silica (SiO.sub.2), and water (H.sub.2 O). The two hydrous calcium silicates that generally are of interest are: tobermorite having the formula 4CaO.5SiO.sub.2.5H.sub.2 O; and xonotlite, having the formula 5CaO.5SiO.sub.2.XH.sub.2 O. Hydrous calcium silicate products often are used as heat insulation materials.
Methods for reacting and drying a molded aqueous slurry of reactive cementitious constituents and reinforcing fibers to form hydrous calcium silicate insulation products are known in the art. One such method includes placing a molded slurry of the reactive cementitious constituents and reinforcing fibers in an autoclave, introducing pressurized saturated steam into the autoclave to indurate the slurry, and removing the products from the autoclave. Another such method includes placing a molded slurry of the reactive cementitious constituents and reinforcing fibers in an autoclave, introducing pressurized saturated steam into the autoclave to indurate the slurry, simultaneously further indurating and drying the slurry with superheated steam to convert the slurry to a final product, reducing the pressure in the autoclave to atmospheric pressure and removing of the product.
In practice the principal slurry constituents, i.e. calcareous and siliceous materials, reinforcing fibers and water are mixed to form a slurry which is then molded to impart a predetermined shape to the slurry and final product. The slurry is molded or shaped in any convenient manner. Generally, however, one of two types of molds is employed, i.e. pan molds or filter press molds. In pan molds, the slurry remains in the mold while the cementitious materials are reacted to convert them to a hydrous calcium silicate insulation. A pan mold generally defines a mold cavity of a particular shape and dimension; e.g. a flat rectangular pan is used to form flat ware or blocks, while an arcuate, generally U-shaped mold forms half-section insulation pieces used to form molded pipe covering for insulating pipes, ducts, and the like. The filter press mold generally comprises a perforated molding surface over which the slurry is poured. A perforated mechanical piston, complementary in shape to the mold, compresses and slurry and dewaters it to the point where it is self-supporting. The filter press molding technique is conducted in the absence of any applied heat and under pressure sufficient only to express water from the slurry in the filter press and to form solid articles such as pipe covering and flat ware. At this point the molded slurry becomes the product defined as "greenware" in this description. The filter press molding technique is described in U.S. Pat. No. 2,699,097.
The greenware is further cured to the desired hydrous calcium silicate product by several methods. One method introduces pressurized, saturated steam into a closed system or autoclave in sufficient amount to bring the pressure in the autoclave to 100 to 350 psi at a temperature ranging from 328.degree.-406.degree. F. in 30 minutes or less and preferably within 15 to 30 minutes after the introduction of the steam begins. (The term "pressure" as used herein refers to gauge pressure (in pounds per square inch), i.e. the pressure above that of the atmosphere.)
The greenware is then maintained under this steam pressure in an autoclave for a period of time sufficient to indurate it. After the product has been removed from the autoclave, it can be dried in an auxiliary drier, if required. The drying temperature in the auxiliary drier must be below the decomposition point of the organic fiber reinforcement to achieve the best possible strength of the product.
The other method disclosed herein introduces pressurized, saturated steam into a closed system or autoclave in sufficient amount to bring the pressure in the autoclave to 100 to 350 psi in 30 minutes or less and preferably within 15 to 30 minutes after the introduction of steam begins. (The term pressure as used herein refers to gauge pressure (in pounds per square inch), i.e. the pressure above that of the atmosphere.) After the introduction of the steam, the temperature in the autoclave is raided by heating coils to a temperature ranging from about 407.degree. to about 600.degree. F. to produce the superheated steam.
The greenware is then maintained in the autoclave until a predetermined percentage of the moisture (by weight) of the ware has been removed by evaporation into the superheated steam atmosphere. The free moisture is reduced substantially during the cycle, but never drops below 10% by weight of solids during the entire cycle. While continuing to circulate the steam in the system, the pressure in the autoclave is reduced to atmospheric conditions within 60 minutes or less and the final product is removed. The product subsequently can be dried in an auxiliary drier, if required. The drying temperatures in the auxiliary drier must be below the decomposition point of the organic fiber reinforcement, or a brittle product is obtained. The retention of at least 10% by weight-free moisture in the molded slurry allows for simultaneous indurating and drying in an autoclave at temperatures above the decomposition point of the reinforcing organic fibers.
An important step in the overall process is the gel formation step prior to molding the slurry. After a mixture of pulverized siliceous and calcareous materials are suspended in an aqueous slurry sufficient time must be allotted for the slurry to gel before it can be processed further. This period of gelation is necessary to permit the formation of a thickened slurry. Gelation requires alternate quiescent periods for allowing the formation of nucleated solids followed by gentle agitation.
I have now discovered that the addition of pulverized greenware material derived from the initial filter press molding of the slurry in the absence of applied heat substantially decreases the amount of time required for this preliminary gelation step.